OCT is a method of interferometry that determines the scattering profile of a sample along the OCT beam. OCT systems can operate in the time domain (TD-OCT) or the frequency domain (FD-OCT). FD-OCT techniques have significant advantages in speed and signal-to-noise ratio as compared to TD-OCT. The spectral information discrimination in FD-OCT is typically accomplished by using a dispersive spectrometer in the detection arm in the case of spectral-domain OCT (SD-OCT) or rapidly scanning a swept laser source in the case of swept-source OCT (SS-OCT).
OCT systems have been used in the prior art to scan the retina of a patient so as to perform medical diagnosis. FIG. 1 shows an embodiment of an OCT scan module for scanning the retina of an eye in the prior art, the OCT scan module illustrated by FIG. 1 comprising fiber 1, collimator lens 2, X/Y scan-unit 3, retina scan lens 4, ocular lens 5, beam splitter 7, internal fixation lens 8 and internal fixation target 9. The solid-line-beam-path in FIG. 1 shows the OCT scan beam path in which a beam is focused on the retina of a patient's eye 6 under investigation to scan the eye 6. The beam can originate from any one of a variety of sources including broadband light sources with short temporal coherence lengths or swept laser sources and can be introduced into the OCT system via fiber 1. The dashed-line-beam-path in FIG. 1 shows the fixation beam path; the internal fixation target 9 is coupled to the OCT scan beam path via the internal fixation lens 8 and the beam splitter 7 and is then imaged on the retina of the eye 6 under investigation via the ocular lens 5 to define the viewing direction of the eye 6 under investigation during retinal scanning. Light scattered from the sample is collected, typically into the same fiber 1 used to route the light for illumination. Collected sample light is combined with reference light (not shown) to form light interference in a detector (not shown). The output from the detector is supplied to a processor (not shown). The results can be stored in the processor or displayed on a display. The processing and storing functions may be localized within the OCT instrument or functions may be performed on an external processing unit to which the collected data is transferred. This unit could be dedicated to data processing or perform other tasks which are quite general and not dedicated to the OCT device.
However, in daily diagnosis, it may be desired to use an OCT system to scan structures in the anterior segment of the eye of a patient such as the cornea. In order to use an OCT system to optionally achieve both retinal scanning and corneal scanning, in the prior art, the portion of the OCT system shown in FIG. 1 is varied to obtain the designs shown in FIGS. 2, 3 and 4.
As shown in FIG. 2, the retina scan lens 4 of the OCT system in FIG. 1 can be replaced with a anterior segment scan lens 10 to switch the OCT system from retinal scanning to anterior segment scanning (such as corneal scanning) without changing the fixation beam path. In order to realize the OCT system that can be used both for retinal scans and anterior segment scans as shown in FIG. 2, there is a need to provide the existing OCT system with additional optical-mechanical modules to exchange the retina scan lens 4 and the anterior segment scan lens 10 and to position different lenses precisely in the optical system of the OCT system and adjust the length of one of the reference arm and the sample arm to achieve the corresponding function. Thus complexity and cost of the instrument is greatly increased in order to achieve the function of anterior segment scanning. An example of such an OCT system is disclosed in U.S. Pat. No. US2007/0291277A1, the disclosures of which are hereby incorporated herein by reference as if set forth in their entirety.
Further as shown in FIG. 3, it is possible to add an anterior segment scan lens 11 externally to the ocular lens 5 of the OCT system of FIG. 1 for changing OCT beams from being focused on the retina to focus on the cornea. Due to addition of the anterior segment scan lens 11, the beams of the internal fixation beam path which were focused on the retina will also be focused on the cornea; preventing the eye under investigation from viewing the original internal fixation target 9 sharply anymore. This problem can be solved by adapting the positions of optical components and/or the fixation target in the internal fixation beam path, so as to re-focus the beams of the internal fixation beam path onto the retina of the eye under investigation, so that the eye under investigation can see the fixation target 9 clearly during anterior segment scanning. Adaptation to the position of the optical components and/or the fixation target is typically done by motors. Although the solution of FIG. 3 enables an existing OCT system for retinal scanning to scan a patient's cornea by arranging the anterior segment scan lens 11 externally to the ocular lens 5, it is necessary to adjust the internal fixation beam path to allow the eye under investigation to see the fixation target 9 clearly during anterior segment scan, so that complexity and cost of the instrument is still increased. An example of such an OCT system is disclosed in U.S. Pat. No. US2012/0140176A1, the disclosures of which are hereby incorporated herein by reference as if set forth in their entirety.
Furthermore, as shown in FIG. 4, an external fixation target 12 may be used. During the anterior segment scan, the patient is looking with the second eye 13 which is not under investigation by the OCT system, onto an external fixation target 12. Since both eyes of a human being share the same viewing direction, the viewing direction of the eye 6 under investigation can be defined by defining the viewing direction of the second eye 13. However, the solution shown in FIG. 4 does not work for patients who have only one functioning (seeing) eye. An example of such an external fixation target and relevant treatment method is disclosed in the document “SPECTRALIS Anterior Segment Module” obtainable from the web site “http://www.heidelbergengineering.com/international/wp-content/uploads/2012/03/SPECTRALIS-ASM_How-to-acquire-perfect-image_EN_web.pdf”.